The invention relates to comparators of the kind commonly used in analog to digital (A/D) converters. It particularly relates to the switching variety of comparator in which an a-c coupled amplifier is chopper stabilized and switched between the two comparator inputs so that the output is responsive to the potential difference between the two inputs. Since the switching is synchronous, the input can respond to d-c and the output is a d-c signal. Since the a-c amplifier can be readily made to have high stable gain, comparator sensitivity is high. Since chopper stabilization is employed, substantially zero offset is available without trimming and drift problems, both thermal and long term, are avoided.
In a copending application by Thomas P. Redfern, Joseph J. Connolly, and Thomas M. Frederiksen, Ser. No. 872,966 filed on Jan. 1, 1978, a PRECISION PLURAL INPUT VOLTAGE AMPLIFIER AND COMPARATOR is disclosed. The conventional switching comparator is discussed and a multiple input pair device disclosed along with means for sense polarity control.
Unfortunately all switching comparators have unavoidable stray or shunt capacitance, much of which is produced by the a-c amplifier coupling capacitor. For most applications, this capacitance can, by careful design, be reduced to very low and acceptable values. As a practical matter such shunt capacitance must be charged (or discharged) in the switching cycle. Thus, the switching cycle must be made long enough to provide adequate charging time. In terms of the comparator d-c input, an input loading current is present except for when the inputs are equal. Where the inputs are substantially different such input loading can present a problem in some circuit designs. In any event, it is desirable to reduce input loading and charging time to whatever extent is practical.